The present invention relates to the oxidation of a butane feed to maleic anhydride.
Oxidation of hydrocarbons to maleic anhydride is well known. Feeds which have been disclosed include benzene, butene, and n-butane. A series of patents to Kerr, including U.S. Pat. Nos. 3,156,705; 3,156,706; 3,156,707; 3,238,254; 3,255,211; 3,255,212; 3,255,213; 3,288,721; 3,351,565 and 3,385,796, discloses vanadium-phosphorus oxide catalysts for oxidation of butene to maleic anhydride.
Friedrichsen et al U.S. Pat. No. 3,478,063 discloses oxidation of olefinically unsaturated hydrocarbons with a catalyst containing vanadium and phosphorus oxides and wherein the amount of phosphorus oxide is at least equal to twice that of the vanadium oxide and wherein the catalyst contains at least one other oxide of chromium, iron, cobalt or nickel, and the catalyst is preferably on a carrier. The patent discloses at Col. 4 that the catalyst may have a surface area from 1 to 100 m.sup.2 /g.
Bergman U.S. Pat. No. 3,293,268 discloses a vanadium-phosphorus oxide catalyst for oxidation of butene to maleic anhydride. Surface area is not disclosed for the catalyst in the Bergman reference. Also, as in the Friedrichsen et al reference, the Bergman catalyst is prepared by an aqueous solution method.
Schneider U.S. Pat. No. 3,864,280 discloses a vanadium-phosphorus mixed oxide catalyst having an intrinsic surface area of 7 to 50 m.sup.2 /g. The Schneider catalyst can be prepared using an organic medium as opposed to an aqueous medium.
The use of recycle of unreacted constituents to a reactor is, of course, well known and frequently is employed in various processes. For example, in the oxidation of ethylene to ethylene oxide, ethylene and air are mixed with recycle gas containing mainly nitrogen and unconverted ethylene, and the mixture is passed over the catalyst. The catalyst is typically contained in tubes in a heat exchanger-type reactor with a boiling cooling fluid on the outside of the tubes to absorb the heat of reaction and control the temperature. Typically the catalyst for the ethylene oxidation is silver oxide on a refractory support, and typical operating conditions include a temperature in the range 392.degree. F. to 572.degree. F. and a pressure of 10 to 30 atmospheres.
Bissot et al, in "Oxidation of Butane to Maleic Anhydride", IEC Vol. 2, No. 1, March 1963, pp. 57-60, disclose that, in a process for conversion of butane to maleic anhydride unreacted butane may be recycled to the reactor. However, Bissot et al prefer to use sequential reaction, with maleic anhydride separation between the reactors, and with unreacted butane from the first reactor being fed to the second reactor, etc.
U.S. Pat. No. 3,904,652 discloses the oxidation of n-butane to maleic anhydride using enriched oxygen and with a recycle stream of reactor effluent which lowers the oxygen concentration in the total feed to the reactor. It is known that explosive mixtures of butane and oxygen exist and that some oxygen concentrations can cause an oxygen-butane-nitrogen mixture to go into the explosive range; see, for example, Bureau of Mines Bulletin 503 (1952), FIG. 35, page 62, and Bureau of Mines Bulletin 627 (1965), FIG. 21, page 23. In U.S. Pat. No. 3,904,652, the reactor feed mixture is kept below explosive (flammable) limits by the addition of an inert gas, e.g., nitrogen, to the enriched oxygen fresh feed.
Butane conversion levels in U.S. Pat. No. 3,904,652 are 30 to 70% per pass. The unconverted butane passes out of the oxidizer reactor as part of the reactor effluent. The effluent is processed to remove maleic anhydride. The maleic anhydride-free effluent is then divided into two parts, a recycle stream which is recycled back to the reactor, and a purge stream which is removed from the system. According to this patent, the oxygen level in the fresh feed is not desired to be below 50%; lower levels are not desired, according to the patent, as the lower levels would require increasing the amount of purge gas to be withdrawn from the recycle stream and hence would increase the unreacted butane loss in the process of said Patent.